Choice A is the correct answer. Apply the half-life concept.

1. Half-lives: 10 years ÷ 5 years = 2 half-lives.
2. After 1st half-life (5 years): $80 \times 0.5 = 40$ grams.
3. After 2nd half-life (10 years): $40 \times 0.5 = 20$ grams.
4. Formula: $A(t) = 80(0.5)^{t/5} = 80(0.5)^2 = 20$.

💡 Strategic Tip: Number of half-lives = elapsed time ÷ half-life period.

Choice B is incorrect because this is after only 1 half-life (5 years). Choice C is incorrect because this would be after 3 half-lives (15 years). Choice D is incorrect because this would be after 4 half-lives (20 years).